digest: tweak CollisionResistance docs (#2044)

Author: newpavlov

digest/src/lib.rs

@@ -293,9 +293,11 @@ pub trait VarOutputCustomized: Sized {
 
 /// Types with a certain collision resistance.
 pub trait CollisionResistance {
-    /// Collision resistance in bytes. This applies to an output size of `CollisionResistance * 2`.
-    /// The collision resistance with a smaller output size is not defined by this trait and is at
-    /// least the given collision resistance with a bigger output.
+    /// Collision resistance in bytes.
+    ///
+    /// This applies to an output size of at least `2 * CollisionResistance` bytes.
+    /// For a smaller output size collision resistance can be usually calculated as
+    /// `min(CollisionResistance, OutputSize / 2)`.
     type CollisionResistance: Unsigned;
 }